Cytochrome c2 catalyzes the transfer of electrons from the cytochrome bc1 complex to light-oxidized photosynthesis reaction center (RC) complexes in the photosynthetic bacterium Rhodobacter sphaeroides. The goal of this project is to characterize structurally a cytochrome c2 isoform, isocyt c2. The two proteins are 44% identical in amino acid sequence. However, in vitro RC complexes are reduced /100-fold slower by isocyt c2. It is likely that the difference in kinetics of RC reduction are brought about by structural differences between the two proteins.